1. Field of the Invention
The present invention is directed to an X-ray diagnostic apparatus for mammography examinations having an arm for an X-ray tube and a subject table and having a compression plate arranged between the X-ray tube and the subject table, the compression plate being connected via a mount to the arm and being displaceable along the arm, the compression plate being connected to the mount such that the compression plate is rotatable relative to the subject table in longitudinal and transverse directions of the plate.
2. Description of the Prior Art
In order to obtain the best possible image resolution of the breast with an optimally low X-ray dose in an exposure in a mammography examination, it is necessary that the compression plate be pressed against the breast under examination and lying against the subject table with a force that is uniformly distributed over the entire breast.
Since the breast is thicker close to the rib cage than at the nipple, an attempt has been made according to U.S. Pat. No. 5,706,327 to solve the problem of achieving uniform distribution of the compression force by rotatably connecting the compression plate to the mount such that the compression plate is rotatable relative to the subject table in the longitudinal direction thereof. This is achieved by the mount of the compression plate proceeding along a part of the two longitudinal sides of the compression plate, and by connecting the compression plate to the mount via shafts that are arranged respectively at the longitudinal sides. When the compression plate is pressed against the breast under examination, the compression plate is rotated around the shafts and therefore presses against both the portion proximate to the rib cage as well as against the portion at the nipple. A disadvantage of this structure is that the parts of the breast that are pressed laterally outwardly are not necessarily pressed together with the same compression force as the parts of the breast that are pressed together first by the compression plate.
An X-ray diagnostic apparatus of the type initially described is disclosed in U.S. Pat. No. 5,506,877. The mount of the compression plate is U-shaped and extends along the transverse side of the compression plate as well as along its two longitudinal sides. The compression plate is rotatably connected to the mount via shafts that are attached to the free ends of the mount. When, using known means, the compression plate and the mount are shifted toward the breast under examination, the compression plate is arranged parallel to the subject table. When the compression plate reaches the breast, it is rotated around the shafts in its longitudinal direction with means that are not described in U.S. Pat. No. 5,506,877 such that the compression plate presses both against the end proximate to the rib cage as well as against the nipple portion. The mount, and thus the compression plate as well, are motor-rotatable in the transverse direction around the center axis of the compression plate. As a result of this possibility of also being able to rotate the compression plate perpendicularly to the center axis, a uniformly distributed pressure over the entire breast can be obtained when the breast is correctly placed on the subject table. Placing the breast correctly means that the imaginary center axis of the breast should proceed along the center axis of the compression plate around which the plate is rotatable. A disadvantage of this known X-ray diagnostic apparatus is that all movements of the compression plate are driven with motors, which makes the apparatus expensive to manufacture.
An object of the present invention is to provide an X-ray examination apparatus for mammography examination of the type initially described having a compression plate that, using simple and thus comparatively inexpensive means, generates a pressure against the breast under examination that is automatically uniformly distributed over the breast in an optimum way.
This object is inventively achieved by connecting the compression plate to the holder with at least one resilient connecting element, at at least one connecting point. With the resilient connecting element, the compression platexe2x80x94given a compression of a breastxe2x80x94can be automatically rotated in the longitudinal and transverse directions without the assistance of motors so that an optimum distribution of the force against the breast is achieved. As a result thereof, a high image resolution with a comparatively low X-ray dose is accomplished in an exposure.
In an extremely simple embodiment of the compression plate of the invention, a single connecting element is arranged in the middle of that side of the compression plate that is directed toward the mount. The aforementioned advantages are achieved as a result, particularly when the breast under examination is placed such that the imaginary center axis of the breast proceeds along or in the proximity of the center axis of the compression plate around which the plate is rotatable.
In a further embodiment of the compression plate of the invention, two connecting elements are arranged at respective sides of the middle of that side of the compression plate directed toward the mount. In addition to the advantages already mentioned, a further advantage is achieved in that the breast need not be brought into a special position relative to the compression plate. The compression plate has no defined rotational axis because the compression plate is connected to the mount in two resilient connecting points spaced from one another.
The mount can be fashioned U-shaped and proceed along one transverse side of the compression plate as well as along the two longitudinal sides thereof, with each longitudinal side of the compression plate being connected to the mount by at least one connecting element. The compression plate is connected to the mount in four points. The other two connecting elements are preferably attached to the transverse side of the compression plate, spaced from one another.
In another, embodiment of the compression plate of the invention, the mount is fashioned U-shaped and proceeds along the one transverse side of the compression plate as well as along both its longitudinal sides, with each longitudinal side of the compression plate connected to the mount with two connecting elements. The connecting elements are preferably arranged in the proximity of the corners of the compression plate. The number of resilient connecting elements and their placement in this embodiment of the compression plate is optimum in view of an automatic distribution of the pressure over the breast during a compression, regardless of where the breast has been placed on the subject table, since the compression plate has a flexible, non-defined rotational axis.
In another embodiment of the invention the mount is provided with at least one rubber bushing and the compression plate is connected to the rubber bushing via a shaft that is secured to the compression plate.
In a further embodiment of the invention the compression plate is provided with at least one rubber bushing and the mount is connected to the rubber bushing via a shaft that is secured to the mount. In conjunction with a compression, each shaft, given a rotation of the compression plate, is shifted in height in the longitudinal and transverse directions at each rubber bushing, so that a deformation of the respective rubber bushing ensues. After a compression ends, each rubber bushing automatically returns into its original shape.